E320 Cooling Problems
 

I have a 95 E320 that contimues to have an overheating problem due to the auxilliary fans not coming on. I have researched this board on the problem and ways to troubleshoot it, and there are some discrepancies that I would like to have cleared up. First, my understanding is that both fans are off or on together (no single fan operation). Next the fans run on low speed when A/C line pressure trips an overpressure switch. The low speed is produced by a resistor which is in that circuit (I assume in series with the fans?). The fan high speed is triggered by a thermo switch located on the thermostat neck. There seems to be some confusion on how to test the thermo switch. Some posters say that merely removing the connector (which produces infinite resistance across the connector) will activate the fans. Other posters have suggested grounding or jumping the connector pins on both the A/C pressure switch or the thermoswitch. Whether it be the overpressure switch on the A/C or the thermoswitch.......do both switchs operate through the SAME relay? There are two connectors on the thermostat housing. One, a two wire connector, while the other appears to have 3 or 4 wires. Which one is the thermoswitch? Lastly, where in the wiring harness is the EASIEST place to jump battery voltage to test each fan? Sorry for the long post, but troubleshooting these fans seems pretty straightforward, but is proving otherwise. I would appreciate it if someone would confirm my understanding and provide some insight to solve my problem.

if it's an overheating problem and the fans don't come on then you have to check the main sensor by the thermostat. note that the fans should come on at around 107 degrees. if you pull the connector off (you are right - infinite resistance) then it should come on. the sensor is the one that is painted blue.

the low speed fan sensor by the AC drier is tested by shunting the terminals. there is a fuse in the fuse box as well as a resistor under the driver side headlight that needs to be checked.

I will add some to Jims post...

There is a mid-production change on the E320, 124.032 chassis fan circuit.
First, there are now 3 fuses to be concerned with.
1- Fuse #7 in the fuse box. -- relay feed.
2- Both fan speed relays are now fused, [ on top of each relay].
These fuses are fan feed [load].
These are located behind fuse box, under panel. [Remove 6 screws]
The one in slot "C" is high speed fan [K9, blue one]. This originally called for 15A., but has been increased to 20.
Slot "B" is low fan resistor relay. This is 30A. [ K10,green relay]
This is resistor series feed for low speed. [ thus the higher Amps for lower speed].
PS - the fan load is always both fans, They are wired parallel.
So, first check is Fuses. The trick of jumpering the a/c high pressure sw. is that if that brings on low fan, you know you have good fuses #7 and relay fuse. Plus you now know your fans themselves are good.
Now we get to the high speed. [ which has nothing to do with low].
This is triggered by the temp thermistor sensor at the thermostat housing when the coolant temp causes the resistance in the sensor to DROP to around 275-300 ohms. This is a negatve temp/resistance type sensor. [ a good test is ohm meter ---- I have a temp/ohm chart , if need be]
This is the blue , 2 wire sensor everyone talks about, BUT, there is a late change-over to a 4 wire/prong sensor in 95.
The early ones had 3 sensors
1- single wire on top of housing- This is gauge sens .
2- 2 wire, blue on top of housing - this is aux. fan sens.
3- 2 wire, 3 prong on side of housing- this is Engine Management control sens.
The late ones have changed over to a 4 wire/prong sensor that now combines both the ECU and Aux fan sensors. The operation is the same, but one needs to know that the pins on the new one for the fan circuit will be pins 2 & 3.
This may add to/ or help clear up some of the confusion.
The main thing to do after checking to see which sensor your E320 uses is to check all fuses and then jumper the a/c sw to see where you stand and go from there with the sensor test.....

<<I will add some to Jims post...>>

Correction--- "Joes post"

Sorry Joe......

Joe and Arthur, thanks for the fast response. Since there are only TWO sensors on the thermostat housing, I guess that I can assume that I have the later version, which means that the aux fans are controlled by the 4 wire sensor. Since the thermoswitch is a thermistor, my assumption is that resistance DROPS as coolant termpature rises, correct? If this be the case, then pulling the sensor connector (which would produce infinite resistance across the pins) will NOT trigger the high speed fans as had been previously reported. If this is the case, then I should be able to jumper the two pins for the aux fans, and they should come on, right? I will pull the relay cover this evening and check fuses and also jumper the A/C switch to confirm that part of the circuit and fan integrity. If that proves out the fans (at low speed), can I jumpter the temp sensor pins to confirm high speed fan operation? Please corrrect me if any of my above assumptions are incorrect. Thanks in advance for the help.

To be honest with you, I could never fiqure out why the high speed comes on when the sensor is disconnected, but I have noticed that this test does not allways work. It seems to work at certain temps and not others.
The resistance value drop is calibrated to turn on a switching device
[ maybe a gate or SCR, etc, - not sure] in the ECU. From there the relay gets the signal. It may be that the disconnect spikes the correct trip - maybe someone knows ???
Anyway, as a precaution ,I always use a 1/4 watt, 180 ohm resistor so I am sure of not screwing with the ecu with a direct short or open circuit.
The 180 is beyond the res/temp range for cut-in, so that seems to work well for the high speed test....

PROBLEM SOLVED!!
 

As you will note from the previous threads, I have been plagued with overheating on my 1995 E320. Thanks to all that responded, I was able to trouble shoot the system, and found a number of problems. The first one that was the most difficult to unravel was that the low speed and high speed relays had been reversed. This may have occured about a year ago when I had the car worked on because the alarm system had immobilized the car, and the mechanic did some wiring changes to bypass the alarm interlock.

Secondly, I apparently had a bad temperature sensor. Not very expensive to replace (about $40).

Lastly, when the above two items were corrected, only ONE of the auxxilliary fans came on! So, I apparently had burned up one of the fans sometime in the past.

Incidentally, unless there is some secret to replacing the aux fans on the 124 chassis.............it's a many step job! Even though the aux fans sit in front of the radiator........you cannot remove them from the front. Instead, you have to remove both the radiator and the A/C condenser.........and the fans then pull out from the rear. This is some terrible German engineering. I had to drain the radiator and depressure the A/C system to get the fans out.

At any rate, the deed is done and the car is running nicely now.

One question for the BB, though. Sometime ago, I recall a poster offering a custom plug with a resistor built in that reduces the trigger temperature for the aux fans from 107 degrees..........down to about 95 degrees. But, I can 't find the information from the archives. Would appreciate any info that would allow me to either fabricate the plug or purchase one. I get a little nervous when the temp gauge inches up to about 110 degrees before the fans come on!

Just as I was looking for more info on my issue I found this link to your need for the fans speed modification..


http://pages.prodigy.net/jforgione/MB_S500.html

same problem
 

The car (95 E300d) usually runs around with the temp indicator at the top of the 8 (gauge) on the freeway and around town, summer or winter. If I turn on the AC it jumps well above the next mark on the gauge for a bit on the freeway, and around town it goes even higher and stays there to the point I think it turns off the AC. If I go up a prolonged grade on the freeway the temp climbs accordingly, but really moves if the AC is on. Freeway driving, over time will get the temp down to the 8 again, AC on or off. Any thoughts? Do have a problem?
I checked the engine driven fan after I parked it, and it is turning. I disconnected the temp sensor (blue plug) in the inlet to the engine block and both fans came on at a very high speed, when connected back the fans turn off. Is there a way to check the temp sensor since for as long as I have owned this car I have never seen those fans turn. I did check the relays, fuses, and resistor as mentioned earlier on thos post, they all seem fine.


Thanks,

B52Rule


__________________

I had a similar issue which vanished once we recored the radiator.

This also occured on my Eurovan which had major issues keeping the engine cool when in town with the AC on, you could hear the secondary fans howling away like mad men, at all times when the AC was running.

Again after a recored rad the issues went away.

If you pulled the connector on the thermo switch and both fans came on, you have essentially proved out the fan circuit including the relay. If the engine is running hot at that time, the culprit is more than likely the thermoswitch. You can measure the resistance across the terminals..........but, to tell you the truth....the switch is very inexpensive.........less than $30. So, I would just replace it.

The part about your note that is confusing has to do with the temp gauge reading hot while driving at speed. At speed, air flow across the radiator should be more than enough to maintain low coolant temperatures. At speed, the viscous fan is freewheeling and the auxilliary fans should be off.

Overheating all the time is probably a defective thermostat...again, this is an inexpensive part to replace.

As another poster indicated, you could have a plugged radiator, though if you are running an anticorrosive antifreeze........plugged radiators are rare.

In summary, you could have more than one thing wrong.......as did I. When I finally sorted out my overheating problem, I found that the hi and lo speed relays had been reversed, the thermoswitch was bad, and one of the two auxilliary fans was defective!

parts
 

I think replacing the thermoswitch and thermostat are in order. I found the thermostat in Fast Lane, but not the switch. I sent in a parts request so we will see what happens.

Thanks,

B52rule

some results........kinda..........
 

If I disconnect the two prong pigtail at the top of the block, key on or off both fans come on high.

If I disconnect the blue with red trace leads to a relay below the drivers headlight the fans never come on. I can ground them out, connect to each other, connect the terminals, or just about any other combination you can think of............no fans at any time.

Any suggestions?

I did check all the fuses BTW.

fixed.............I think
 

The 30 amp in the relay fuse is good, but the 15 amp fuse in the relay was blown, I replaced it and the fans come on at low speed, intermittently while the AC is running. This happened just after I fired it up after replacing the fuse. I'll take it out tomorrow and we will see what really happens.
Where is this resistor located?

B52Rule


__________________

<<The 30 amp in the relay fuse is good, but the 15 amp fuse in the relay was blown, I replaced it and the fans come on at low speed, intermittently while the AC is running.>>

The reason the low [ a/c fan] was blown is b/c someone has switched relays or fuses..

The low fan is relay K10- and belongs in slot "B"... that is a 30A.,
not a 15 [ which will blow again]
The high fan relay K9 is in slot "C" and originally had a 15 , but
Benz has allowed a 25A replacement..

The dropping resistor for K10 is behind the left headlamp, but if you now have low a/c fan, the resistor is fine ...

Your problem for no fan was bad fuse..and under-rating was the possible cause of failure..
{ I am assuming chassis 124.131]

more info....
 

In the "B" slot there was a Blue relay, and calls for a 15 amp fuse, part # 0025420119. In the "C" slot there was a Green relay, and calls for a 30 amp fuse, part # 0015429619. The slots were marked with a white letter under the relays. They were switched and I have now put them back as you suggested. I'll replace the 15 amp with a 25 as soon as I get home. You are correct, it is a 124.131 chassis and a 606.910 engine. Thanks for the help, I'll let you know what happens.

Thanks,

B52rule

In the "B" slot there was a Blue relay, and calls for a 15 amp fuse, part # 0025420119. In the "C" slot there was a Green relay, and calls for a 30 amp fuse, part # 0015429619. The slots were marked with a white letter under the relays. They were switched and I have now put them back as you suggested. >>


They are inter-changeable, so I see this happen quite often..
Usually happens when someone is testing relays and changes them , then forgets what goes where...
They mistakenly switch the slots... resulting in repeat fuse failure of a/c
low fan circuit/fuse.....
Recommend 20/25 A for low fan [ Benz approved this change],
as the 15 was found to be borderline on low fan circuit..
30A on coolant temp high fan.

progress
 

It's all bolted up with the relays in the right places and correct amp fuses. Interesting to note, I bought this as a Starmark vehicle 1999 from one of the North County dealers here in San Diego, had it back for several issues, none of them for cooling however. When I did mention that it had these symptoms I was told it was normal operations. It has never had the A/C system worked on while I have owned it. Nothing more than routine maintenance here, oil, filters, coolant etc. I'll run it for a few days any let you know the results.

Thanks,

B52rule

Recently my '95 E320 wagon was running warmer than usual and I finally realized my low speed fans were not coming on. Thanks to this thread, I found the problem.

I've owned this car since 2001 and as far as I know, no one has worked on the a/c system during this time. But sure enough, the relays were swapped and the 15A fuse was blown. Thanks for helping me fix it!

<I've owned this car since 2001 and as far as I know, no one has worked on the a/c system during this time. But sure enough, the relays were swapped and the 15A fuse was blown. Thanks for helping me fix it!
__________________>>

Glen
Double check the relay positions b/c that old thread was incorrect on locations depending on V8 or IL-6 model.

On you chassis , the blue relay goes in slot B and is low fan, [ K10] with high fan being slot C. [K9]
So, slot C stays with a 30A fuse and slot B can be increased to 20/25 for low fan [ a/c activated]. They found the orig 15A did not hold , specially when they came out with the Dual Fans set-up.
The confusion on that old thread was we did not realize the difference in the two models [ 124.032/124.036]
It was later changed in the archieves and I edited that one for future use.
It is easy to verify the correct locations by activating the high fan by pulling the plug on the blu 2 wire sensor at the therm housing ..that defaults HIGH fan ..when that high fan comes ON , simply pull the green/30A relay from it's slot..if the fan still runs , you know you have the relays backwards [ wrong slots]...sorry for the confusion.

Thanks Arthur, I'll verify tonight. Just so we're clear, this is for my '95 W124, E320 wagon...3.2 liter I6. It's a late car so has the 4-prong thermo sensor.

Yes ..the later went to the 4 prong , but that is still the same ..they just put 2 sensors into one casing , eliminatingthe 2 wire blu sensor...same deal..if you unplug that sensor with the car key On, the high fan will come on [ that is how we check high fan circuit]..when that fan is now On, it will quit when you pull the high fan relay, which should be the 30A/Green relay...if NO, the relays are in the opposite slots as they are supposed to be,and you will never know that b/c everything still works, but you will have the wrong fusing.. If the car was the V8 , the relay slots are different.
Let me know when you check it.

Another way is to jumper the sensor at the a/c drier that has the 6" pigtail wires on it..when you jumper them , the LOW fan comes ON ..you then pull the blu relay and that fan should stop if the relay is in the correct slot...either way , both test will tell the same tale and are simple , quicky tests.

Ok, I finally had a few minutes to verify everything. In my '95 E320 wagon, the high speed fans are controlled by the GREEN relay with 30A fuse in slot C, which is closer to the front of the car. The low speed fans are controlled by the BLUE relay, now with a 20A fuse. This relay is in slot B, which is towards the rear in relation to the GREEN relay.

I verified the high speed relay by disconnecting the 4-prong thermo sensor which caused the fans to run at high speed, then I pulled the GREEN/30A relay and they stopped.

Thanks again for your help!

Very Good..

You're Welcome.

Arthur, you must be getting pretty close to induction into the automotive Hall of Fame by now. Or is it sainthood? Something was mentioned early in this thread that I've puzzled over for a long time. Since I'm still a bit shaky on the finer points of electric circuits, help me out with this: the OP said that pulling the thermoswitch (blue CTS) to default hi fan was the result of creating infinite resistance across the sensor connector. But if that sensor is a neg temp coefficient resistor, it seems like current shouldn't flow to the hi fan until resistance goes DOWN far enough, not goes INFINITE. What (else) am I missing here?

<

Arthur, you must be getting pretty close to induction into the automotive Hall of Fame by now>

Couple of more weeks...............:)

The Default fan is open circuit detection by N22 module and it is an engine over-heat protection circuit so you don't blow the engine if that sensor circuit has a failure....it has nothing to do with the ohms value of that sensor..it is triggered when it it sees open circuit/failure anywhere in the temp INPUT circuit for the N22 module and that includes wiring harness, connectors, and sensor..
So , if sensor fails, wiring fails , open circuits , etc...the high fan Defaults to protect the engine in the event that system is not working at the same time an over-heat condition exist..we just use that info to trick the N22 module into thinking there is an OPEN circuit by unplugging the sensor so we can Verify the high fan circuit w/o getting out meters and schematics, etc. It is just another Trick Of the Trade to accomplish a quick, positive diagnosis..
Same deal with the low fan ..we know that you can simply fool the cut-in relay for low fan for a/c by jumping the high side pressure sw. to verify the low fan circuit. And these test take nano-secounds to do and cuz we are a lazy sorts..we do not do all the complicated checking you see posters mention..we just go to the heart of the diagnosis with our bag of tricks.

It's been a few days, any update?
Thanks,

94 E320
 

Hello,

Im bringing this thread back to life since i've been plagued with the same problem of the engine over heating.

The aux fans do not turn on at all. And there is only one fan speed.

I've read all the posts and so far this is what i've tried...

Pulled the blue switch with the car turned on. No change in fan speed noted.
I opened up the fuse box to check the relays. the Green fuse is in slot C. and the Blue fuse in slot B. also of note is the Blue fuse seems to be tuned 90 degrees from the orientation of the other fuses.(not sure if this means anything)


I apologize for my ignorance... I am new at this but i'm willing to learn.

Thanks!

JM124, the way you've stated your problem, leads me to wonder if you may be confusing your 2 auxiliary fans with the clutch fan (the single, main engine fan between radiator and engine block). Is that possible? Your aux fans (the 2 fans at the front of the car immediately behind the grille) do have two speeds. When you pull the connector off the blue sensor (key on, cool engine), that should give you hi speed on the 2 aux fans. Does that happen?

124 m104 cooling issues
 

I'm just now coming to the conclusion of my m104 cooling issues, this on a 93 300TE with the same m104 engine as your car (I also have a 94 e320 m104). Rather than recapitulating, here is the link to the forum thread:

http://mbca.cartama.net/showthread.php?t=30017

I don't go into the twin electric fans much, but certainly every other aspect of the m104 colling system is touched on. Maybe you'll find something of value in there based on my experiences.

Cheers, JAS

Thanks Jas2wa.

Just ot answer your question Cal. No thw 2 aux fans do not come on at all. even when the blue switch is pulled.

Check fuse 7. Try jumpering the pigtail leads from the red pressure switch at the A/C drier (key on engine off). That should give lo aux fans. If no fans, tap on the fan hubs to make sure they're not stuck. If still no, bring jumper from battery + to the two-wire side of the series resistor located behind the driver side headlight. That bypasses all of the aux fan circuit except the wiring between the resistor and the fan motors. If still no, the wire between is bad or both fan motors are bad. If that turns on the fans, you've verified that the fan motors are good, the problem is upstream in the circuit. Check the blue and green relays that each have a fuse on top that are discussed earlier in this thread. I think the relays are located in the relay box at the back of the main fuse box. These relays power the lo and hi fans.

<. Try jumpering the pigtail leads from the red pressure switch at the A/C drier (key on engine off). That should give lo aux fans. If no fans, tap on the fan hubs to make sure they're not stuck. If still no, bring jumper from battery + to the two-wire side of the series resistor located behind the driver side headlight. That bypasses all of the aux fan circuit except the wiring between the resistor and the fan motors. If still no, the wire between is bad or both fan motors are bad. If that turns on the fans, you've verified that the fan motors are good, the problem is upstream in the circuit.>>

I know a guy here that writes instructions just like that...

Yeah, I was paying attention when you described it the first 3,000 times. :)

That's how many times it takes.

I swiched the relays as they were reversed.

So ONE of the fans started to work after tapping it a few times. But a few hours later it doesn't turn at all anymore!?

wouldn't you know...
 

I have come full circle with my cooling issues. Per the dealer, the high pressure switch that turns on the twin electric fans is bad, and the fans themselves are toast, too. One turns (but apparently not fast enough) and the other is dead.

$700 for the entire twin fan assembly??

Are these indivudual fan units replaceable themselves (for what looks to be about 140/each)?

Yes. P/N 000-500-85-93 available for about $250 each from dealer or about half that from online vendor.